TPA is the true raw material for production of CHDM and CHDA. Hydrogenation of the aromatic ring of TPA would give CHDA and hydrogenation of the aromatic ring as well as the carboxylic acid side-chains would give CHDM. This is as shown below:TPA+3H2→CHDACHDA+4H2→CHDM+2 H2OHowever, direct use of TPA in hydrogenation reactions is made complicated by its high melting point and poor solubility in reaction solvents (especially in water) at temperatures that are favorable to the hydrogenation reactions. As a result, instead of TPA, industrial processes for production of CHDM and CHDA rely on hydrogenation of derivatives of TPA that can be more easily used in liquid phase—in melt or in solution. This means that such processes have to include additional processing steps for first converting TPA to its desired derivatives—typically requiring high initial investment, increased energy consumption, and high operating costs.
For example, use of dimethyl terephthalate (a diester of TPA, hereinafter referred to as DMT), for production of CHDM is well known in prior art. Hydrogenation of DMT to CHDM typically proceeds in two stages—hydrogenation of the aromatic ring over palladium catalyst to get dimethyl 1,4-cyclohexanedicarboxylate (hereinafter referred to as DMCD) followed by hydrogenation of the ester side-chains of DMCD over copper and chromium based catalyst to get CHDM. The two reactions cannot be combined into one stage and the sequence of reaction steps cannot be changed as significant by-product formation occurs otherwise. Both stages can be carried out in gas-liquid-solid (hereinafter referred to as GLS) reactors as described, for example, in U.S. Pat. No. 3,334,149 (assigned to Eastman Kodak Company) and in U.S. Pat. No. 6,187,968 (assigned to SK NJC Co., Ltd.). The hydrogenation of DMCD can be carried out in vapor phase as described in U.S. Pat. Nos. 5,387,752 and 5,395,987 (assigned to Eastman Chemical Company). Preparation of DMT involves esterification of TPA with methanol at high pressures and temperatures that requires specialized, costly process equipment and also results in increased energy consumption and additional operating costs. Additionally, DMT must be separated and purified by distillation, to remove any byproducts and esterification catalysts, prior to its use in the hydrogenation stages. Also, hydrogenation of DMT creates methanol. Additional processing steps are therefore required for processing and purification of methanol so that it can be reused in the esterification process.
Process for producing CHDM from TPA is claimed in U.S. Pat. No. 8,410,318 (assigned to Eastman Chemical Company). However, this is highly misleading as the first stage of the claimed process involves esterification of TPA with (4-methylcyclohexyl) methanol (hereinafter referred to as MCHM) to produce the bis(4-methylcyclohexyl)methanol diester of TPA. This diester is then hydrogenated in two additional stages to CHDM. This in essence is similar to the DMT based process (the only difference being the use of MCHM instead of methanol to form the initial diester of TPA) and therefore suffers from the same aforementioned problems.
Processes for the production of CHDA using derivatives of TPA as feedstock are also known in the prior art. For example, U.S. Pat. Nos. 5,118,841 and 5,202,475 (assigned to Eastman Chemical Company) describe hydrogenation over ruthenium catalyst using aqueous solution of sodium salt of TPA. These processes not only require the additional step of preparing such salts, but also require treatment of the hydrogenation product with mineral acids in order to recover the CHDA product. Additionally, the sodium salts of mineral acids generated in the process also need to be disposed of.
Process for producing CHDM from CHDA or esters of CHDA is described in U.S. Pat. No. 6,294,703 (assigned to Mitsubishi Chemical Company). CHDM is produced by hydrogenation of aqueous solution of CHDA or diesters of CHDA in GLS reactors over catalyst comprising ruthenium, tin, and platinum at temperatures from 150-240° C. and pressures ranging 1-25 MPa. The amount of water used is preferably 1 to 10 fold by weight of the CHDA feed. This patent provides a route to CHDM starting from CHDA, but assumes the availability of CHDA and does not address the problems associated with production of CHDA from TPA or otherwise as noted above.
Based on this, it is clearly evident that it will be beneficial to have processes for production of CHDM and CHDA directly from TPA without going through an intermediate TPA derivative. As noted initially, the main hurdle for such processes is getting TPA into the liquid phase. TPA does not melt even at very high temperatures (sublimates above 300° C.) and its solubility in suitable reaction solvents is very poor. For example, solubility of TPA in water and the amount of water required for complete dissolution of TPA are listed below (water is a natural choice for reaction solvent as it is also a product of the hydrogenation of carboxylic acid side-chains):
TemperatureTPA SolubilityWater for Complete Dissolution(° C.)(wt fraction)(wt Water:wt TPA)1500.0022456:1 2000.01661:12500.11 9:1
Note that solubility increases as temperature increases. However, temperatures greater than 200° C. are highly undesirable due to significant by-product formation through decarboxylation and decarbonylation reactions.
Hydrogenation of aqueous slurries of phthalic acids to cyclohexanedicarboxylic acids has been studied in prior art. U.S. Pat. No. 4,754,064 (assigned to Amoco Corporation) describes use of rhodium catalyst in presence of recycled product and U.S. Pat. No. 6,291,706 (assigned to Eastman Chemical Company) describes the use of palladium catalyst. However, such processes as described are limited to batch operation such that TPA solids gradually dissolve in the solution just as the dissolved TPA gets hydrogenated to CHDA. Extending them to continuous operation would require the use of slurry GLS reactors with TPA as well as the catalyst in the solid phase. This is extremely problematic because high residence times and large equipment sizes are required, dissolution of TPA is not guaranteed even at high residence times, expensive filters are needed for separation of catalyst from reaction product, and TPA particles can block catalyst pores and also increase the attrition.
Continuous operation of TPA hydrogenation using fixed beds of catalyst is possible only if TPA is completely dissolved in the solvent. Due to its poor solubility, this would mean that the amount of solvent required is several hundred times the amount of TPA by weight. Reaction mixtures with such large amounts of solvent make the equipment sizes, catalysts requirements, and initial investment prohibitively high. Also, due to high purity requirements and stereo-isomeric nature of the CHDA and CHDM (this is explained below), the hydrogenation product eventually has to be completely separated from the solvent. This leads to very high energy costs, especially when water is the solvent due to high heat of vaporization of water.
U.S. Pat. No. 6,541,662 (assigned to Mitsubishi Gas Chemical Company, Inc.) describes a continuous process for producing a hydrogenation product of an aromatic carboxylic acid such that the aromatic carboxylic acid is dissolved in the solvent by recycling a portion of the reaction liquid. For hydrogenation of TPA to CHDA, the process claimed in this patent is not very useful for the following two reasons. First, no discovery is made in this patent as to the relationship between the relative use of solvent and product recirculation, and as a result, the total solvent flows required (as described in the examples) are still high and would lead to high initial costs and high energy costs for separation. Second, CHDA produced in accordance to this process will contain only 20 to 35% of the desirable trans isomer, and as a result, would require a separate and additional isomerization process (this is explained below) to enrich the trans content.
Based on this discussion, it is clearly evident that viable and practical process options do not exist for continuous production of CHDM and CHDA directly from TPA. Further developments and improved processes are needed, specifically to improve the solvent usage and therefore reduce the associated initial investment and energy usage while producing products rich in the desired trans isomers.
As mentioned earlier, isomeric compositions of CHDM and CHDA are important in polymer applications. High trans content is required to achieve many of the desirable end-use properties. For both CHDM and CHDA, isomerization requires bond-breaking and cannot be achieved simply through bond rotation. CHDA exhibits Lewis-acid catalyzed isomerization that proceeds through the enol formation mechanism. In the melt (above the melting point of trans CHDA), the mechanism is self-catalyzed and will lead to an equilibrium mixture containing ˜66% trans CHDA. This however does not happen in the aqueous phase. For CHDM, isomerization is more complicated due to the —CH2OH side-chains. Metal alkoxide or hydroxide catalysts and severe reaction conditions are required and the yields are often poor.
Isomerization processes are known in the prior art. U.S. Pat. No. 2,917,549 (assigned to Eastman Kodak Company) discloses a process in which cis-CHDM is isomerized to trans-CHDM at temperatures in excess of 200° C. over metal alkoxide catalysts. U.S. Pat. No. 4,999,090 (assigned to Towa Chemical Industry Co., Ltd.) describes a process for isomerization of cis-CHDM to trans-CHDM in presence of alkali metal hydroxide or alkoxide catalysts. U.S. Pat. No. 7,595,423 (assigned to Mitsubishi Chemical Corporation) describes a process for isomerization of cis-CHDA to trans-CHDA that takes advantage of the melt-phase reaction mechanism described above. However, such isomerization processes essentially need to be carried out after separation of CHDA and CHDM as a product. Therefore they represent additional processing steps which lead to higher initial investment and higher operating costs, in addition to their inherent problems and difficulties. It is therefore beneficial to avoid the use and the need for such processes by obtaining product with desired cis/trans composition from the hydrogenation itself.
Industrial polymer applications require 65 to 70% trans content. The prior art processes for producing CHDM referenced earlier are capable of producing CHDM product with this trans content. However, the prior art processes for producing CHDA give CHDA with trans content of only about 20 to 35%. It will be greatly beneficial and highly desirable to obtain CHDA with higher trans content without the need for additional isomerization steps.
We should also note that presence of both cis and trans isomers in the hydrogenation products restricts the methods that can be used for product separation such that all of the cis and trans products need to be separated. Only a part of cis or a part of trans cannot be removed without the need for isomerization as it would mean accumulation of the other isomer in the process. For example, the use of crystallization and solid-liquid separation to separate only the trans product is not possible if we recycle the mother liquor rich in the cis isomer back to the process.
Based on this discussion on isomerization, it is clearly evident that any process for manufacture of CHDM should produce CHDM with trans content of at least 65-70% and any process for manufacture of CHDA should produce CHDA with trans content as high as possible.
Thus, further developments are needed.